There are many known power assist steering systems for automotive vehicles. Some systems provide assist by using hydraulic power, electric power or a combination thereof.
Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (i) apply rotary force to a steering input shaft connected to a pinion gear, or (ii) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (i) a driver's input torque applied to the vehicle steering wheel and/or (ii) vehicle speed.
U.S. Pat. No. 4,415,054, utilizes an electric assist motor having a rotatable armature drivingly connected to a ball-nut assembly which encircles a thread portion of a steering member having rack teeth thereon. The steering member is steerably connected to the vehicle's dirigible wheels. The rotary electric motor is energized in response to torque applied to the steering wheel. The ball-nut assembly converts rotary motion of the electric assist motor armature into linear movement of the steering member which, in turn, assists in steering the vehicle.
Known electric assist steering systems having a ball-nut drive assembly surrounding a steering member and driven by an electric motor have included a power assist steering assembly housing with a steering member and ball-nut assembly radially supported relative to the housing at four locations. The steering member is axially movable relative to the housing and the ball-nut assembly is both axially and radially fixed relative to the housing. When the ball-nut assembly is held radially fixed relative to the housing, assembly of the steering gear parts is difficult and requires extreme manufacturing tolerances to insure proper alignment of parts and efficient operation of the ball-nut assembly.
Also, in such known power assist steering systems having four radial supports for the steering member and the ball-nut assembly, loading of the steering member during use results in bending of the steering member which, in turn, exerts a radially outward force against the ball-nut assembly. Since the ball-nut assembly is radially fixed relative to the steering housing in previous designs, the ball-nut assembly could operate in an undesirable and inefficient manner when under such a radially exerted force.
An object of the present invention is to provide a power assist steering system having a ball-nut drive assembly that permits easy alignment of the steering assembly parts without resorting to extreme manufacturing tolerance control. The present invention is to further provide a power assist steering system having a ball-nut drive assembly that is mounted so as to restrict axially movement relative to the housing while enabling limited radial movement of the ball-nut drive assembly to accommodate radial movement of the steering member thereby insuring proper and efficient operation of the ball-nut drive assembly.